Combining GNAT with LLVM

https://blog.adacore.com/combining-gnat-with-llvm

by samuell • 7 years ago

64 29 7 years ago

blog.adacore.com

7thaccount5• 7 years ago

In theory would the performance be better or machine code smaller?

sanxiyn• 7 years ago

No.

0xDEEPFAC2• 7 years ago

I wouldn't be so sure. Given that this is a new backend for GNAT and the first one is close to 25 years old I assume it will have more modern optimizations and experience behind it - presumably leading to something faster.

Glacia2• 7 years ago

It would be interesting to see comparisons, since afaik it's the only compiler front end that has llvm and gcc back end.

lbenes• 7 years ago

a natural question would be "why are you starting a GNAT LLVM project from scratch instead of building on top of DragonEgg?". If you want to know the answer, check the file README.dragonegg in the repository!

Correct me if I'm wrong, but from the README [1], the answer appears to be:

The dragonegg plugin works with gcc 4.5, 4.6, 4.7 or 4.8

But a little more digging reveals that there are patches so[2]

DragonEgg works for for GCC v8.x

Lack of Google-fu or am I missing something? If they believed dragonegg is flawed, why not just lay it out? I prefer that over a goose chase that doesn't fully check out.

[1] http://llvm.org/svn/llvm-project/dragonegg/trunk/README

[2] https://lists.llvm.org/pipermail/llvm-dev/2017-August/116705...

mCOLlSVIxp6c• 7 years ago

Arguments against DragonEgg are laid out in the gnat-llvm repo here: https://github.com/AdaCore/gnat-llvm/blob/master/README.drag...

jcranmer• 7 years ago

Dragonegg was never a particularly well-maintained project. You're building a bridge between GCC's and LLVM's IR, which means you're highly susceptible to changes in one or the other, and the conversion tends to drop annotations (such as debugging info!). Most people only really cared about Dragonegg as a way to get Fortran code compiled to LLVM IR. Now that Flang exists, the Fortran motivation isn't there.

Essentially, Dragonegg was only ever a last-resort way to get LLVM IR. If you're trying to build a LLVM backend, you might as well emit LLVM IR directly from your frontend rather than trying to deal with a GIMPLE backend and then a GIMPLE-to-LLVM converter.

pyler• 7 years ago

Ada-C++ LTO?

pjmlp3• 7 years ago

Green Hills Ada does it.

https://www.ghs.com/products/AdaMULTI_IDE.html

thesuperbigfrog• 7 years ago

There are many advantages of porting GNAT to an LLVM-based backend:

1) Clean-up of backend interface code to make it more robust and less GCC-specific

2) Allow for GNAT to target more platforms such as modern Apple devices (where GCC support is waning and LLVM is preferred) and WebAssembly

Right now they are stating that it is not production-ready, but I would not be surprised if the LLVM-based GNAT gains peer status with the GCC-based GNAT or even overtakes it.

andikleen7• 7 years ago

They already have 5 others, according to the article. There's nothing gcc specific about it.

7thaccount5• 7 years ago

The main GNAT compiler that people actually use is based on GCC

protomyth3• 7 years ago

GPLv3 license, so the flexibility of how you can include clang in your own tools won't be available for Ada.

saagarjha• 7 years ago

No runtime license exception?

fiddlerwoaroof• 7 years ago

It's complicated, I believe the way it works is that the Community Edition you can download from the Adacore website doesn't have the runtime exception but, if it's built from the code in the FSF's repo, it does. This is Adacore's scheme to make sure that people making money off Ada buy a license.

protomyth3• 7 years ago

Doesn't say: We have just published the GNAT LLVM tool sources licensed under GPLv3 on GitHub for hobbyists and researchers to experiment with it and give us their feedback.

Glacia2• 7 years ago

It pulls runtime from FSF, which has GCC Runtime Library Exception.

ajxs• 7 years ago

I'm very interested in seeing where this leads. I've had so many problems creating Ada capable GCC cross-compilers from the FSF source, and it looks like I'm not alone in this area. This might open up new and better possibilities for platform support. Great work!

Combining GNAT with LLVM

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